1. Field of the Invention
The present invention relates to a projection optical system which is employed when a pattern such as an electric circuit pattern drawn on a projection original plate including a reticle or a mask is transferred onto a photosensitive substrate such as a semiconductor wafer or a glass plate coated with photosensitive material by projection photolithography.
2. Related Background Art
Recently, a projection exposure method is fairly generally employed for transferring a necessary pattern onto an integrated circuit such as an IC or a LSI etc., a flat display of such as a liquid crystal etc.
Especially, for manufacturing a semiconductor integrated circuit or a substrate packaging a semiconductor chip therein, a pattern thereof is increasingly miniaturized and a wider projection area is required for a flat display for liquid crystals, or the like. Consequently, an exposure apparatus, especially a projection optical system thereof, for printing such patterns is required to have a higher resolving power and a wider exposure area.
However, any of projection optical systems conventionally employed in an exposure apparatus does not fully satisfy both of such requirements, i.e., a higher resolving power and a wider exposure area.
More specifically, for obtaining a higher resolving power, it is required to enlarge the numerical aperture of the optical system, which inevitably results in an enlarged lens size. In the same manner, in order to obtain a wider exposure area, the lens size is still enlarged since a flat object is to be projected on a flat surface. If the lens size is enlarged, a glass material for the lens is required to have a larger size. However, it becomes difficult to prepare a glass material having a lager size than the current one in terms of the homogeneity of the material, or the like. An enlarged lens size makes another difficulty in a step of polishing the glass material, and it becomes impossible to polish a lens having a larger size than the present one. Under such circumstances, it is an important object to be achieved up to now to reduce the maximum effective diameter of lens of the optical system, while securing a large numerical aperture.
An object of the present invention is to provide a projection optical system which has a large numerical aperture and a satisfactorily reduced maximum effective diameter of lens of the optical system.
The present invention has been contrived to solve the above-mentioned problem. According to the present invention, there is provided a projection optical system for projecting an image on a first surface onto a second surface, comprises first lens group G1 of positive refracting power including two or more positive lenses, a second lens group G2 of negative refracting power including two or more negative lenses, a third lens group G3 of positive refracting power including three or more positive lenses, a fourth lens group G4 of negative refracting power including two or more negative lenses, and a fifth lens group G5 of positive refracting power including at least six or more consecutive positive lenses, in the named order from the first surface side to the second surface side, wherein either one of the fourth lens group G4 and the fifth lens group G5 has one aspherical surface, the fifth lens group G5 has an aperture stop inside thereof, a portion at which a light flux is diverged right before the aperture stop has a first air lens LA of negative refracting power, the radius of curvature rA1 of the lens surface on the first surface side of the first air lens LA is positive, and a portion at which the light flux is converged at the rear of the aperture stop has a second air lens LB of negative refracting power.
In the projection optical system of the present invention, an aspherical surface is introduced to correct a spherical aberration which is generated due to an enlargement of the numerical aperture. This aspherical surface is to be applied to a portion at which mainly spherical aberration is generated and, more naturally, to a portion at which the spherical aberration can be easily corrected. Consequently, the aspherical surface is to be applied to a portion in the vicinity of the aperture stop. According to the present invention, since the aperture stop is provided in the fifth lens group G5, the aspherical surface is to be applied to the fifth lens group G5 which has this aperture stop therein or to the fourth lens group G4 near the aperture stop.
However, it is more preferable that the aspherical surface should be applied to the portion at which the light flux is converged, in order to avoid the aspherical lens surface from being enlarged. Consequently, in first and second embodiments described below, one surface out of the fourth lens group G4 is selected as the aspherical surface.
Since one of the surfaces is used as the aspherical surface, it is possible to correct a spherical aberration. However, it is also required to correct other aberrations which may be generated due to an enlargement of the numerical aperture.
The projection optical system has not only a high numerical aperture but also a large field size, so that there are generated larger aberrations around the image field. Especially, a coma aberration around the image field is generated to be larger due to the enlargement of the numerical aperture. Further, an amount of a generated coma is normally different depending of an amount of enlargement of the field angle (i.e., increase of the image height). This is called a field angle fluctuation component of a coma aberration.
In order to remove this field angle fluctuation component of the coma aberration, a more complicated correction is required to be conducted. In general, at least another two aspherical surfaces are required for correcting an upper coma and a lower coma, respectively. However, employment of a large number of aspherical surfaces brings about an increase in the cost undesirably. Then, according to the present invention, the field angle fluctuation component of the coma aberration is corrected by a spherical surface, a method of which will be described below.
Generally, in an optical system, the smaller an angle at which a light beam is incident onto each lens surface is, the less aberrations are generated and the more loosen a tolerance or the like becomes, appropriately. Especially, such tendency is strong in an optical system which pursuits the extreme performance of a projection optical system or the like.
However, according to the present invention, there are provided surfaces acting against the light beam to make an angle of incidence large, conversely. These surfaces are a lens surface rA1 on the first surface side of the first air lens LA and a lens surface rB2 on the second surface side of the second air lens LB. Since these air lenses LA, LB are provided in the portions in which the light flux is diverged and the light flux is converged to sandwich the aperture stop therebetween, so that the field angle fluctuation components of the upper coma and the lower coma can be corrected.
Though a considerable amount of aberrations is normally generated on such surfaces oriented to act against light fluxes, converse aberrations are caused by the curved surfaces existing in front or rear thereof and having a similar curvature, that is, the lens surface rA2 on the second surface side of the first air lens LA and the lens surface rB1 on the first surface side of the second air lens LB, so that high order aberrations are corrected by a difference therebetween.
With the above arrangement, the field angle fluctuation component of coma is corrected. As a result, it becomes possible to reduce the maximum effective diameter of lens.
Next, according to the present invention, it is preferable to satisfy the following conditions:
0.1 less than D/L less than 0.3;xe2x80x83xe2x80x83(1)
|PAxe2x88x92PB|xc3x97L less than 1.0;xe2x80x83xe2x80x83(2)
xe2x80x830.2 less than |PA|xc3x97L less than 2.0;xe2x80x83xe2x80x83(3)
0.2 less than |PB|xc3x97L less than 2.0; andxe2x80x83xe2x80x83(4)
0.01 less than Y/L less than 0.02,xe2x80x83xe2x80x83(5)
where
D=tan xcex8xc3x97f5;
xcex8=sin xe2x88x921[NA/n1];
NA: the image side maximum numerical aperture;
nI: the index of refraction of a medium which fills a space between the final lens surface and the second surface;
f5: the focal length of the fifth lens group;
L: the distance from the first surface to the second surface;
PA: the refracting power of the first air lens LA;
PB: the refracting power of the second air lens LB; and
Y: the maximum image height.
Since D provides an almost maximum effective diameter of lens in the condition (1), the condition (1) defines an appropriate range for the maximum effective diameter of lens on the basis of the distance L between the first surface and the second surface. Below the lower limit of the condition (1), the maximum effective diameter of lens becomes smaller, but a satisfactorily large numerical aperture can not be obtained. Conversely, above the upper limit of the condition (1), the maximum effective diameter of lens becomes excessively large, which requires a larger amount of the glass material, resulting in an increase in the cost.
The condition (2) provides a difference between the refracting power PA of the first air lens and the refracting power PB of the second air lens on the basis of the distance L between the first surface and the second surface. Above the upper limit of the condition (2), there is too large difference generated between the refracting powers of the both air lenses, so that the field angle fluctuation components of the upper coma and the lower coma can not be corrected at a time.
Note that the refracting powers PA, PB of the first and second air lenses are defined as follows:
PA=(nA1xe2x88x921)/rA1+(1xe2x88x92nA2)/rA2;
PB=(nB1xe2x88x921)/rB1+(1xe2x88x92nB2)/rB2;
nA1: the index of refraction of a medium on the first surface side of the first air lens LA;
rA1: the radius of curvature on the first surface side of the first air lens LA;
nA2: the index of refraction of a medium on the second surface side of the first air lens LA;
rA2: the radius of curvature on the second surface side of the first air lens LA;
nB1: the index of refraction of a medium on the first surface side of the second air lens LB;
rB1: the radius of curvature on the first surface side of the second air lens LB;
nB2: the index of refraction of a medium on the second surface side of the second air lens LB; and
rB2: the radius of curvature on the second surface side of the second air lens LB.
The conditions (3) and (4) respectively provide the refracting powers PA, PB of the first and second air lenses on the basis of the distance L between the first surface and the second surface. Below the lower limit of the condition (3) or (4), the field angle fluctuation component of the upper coma or the lower coma can not be fully corrected. Conversely, above the upper limit of the condition (3) or (4), a curvature difference between the lens surfaces on the entrance side and on the exit side of the first or second air lens becomes too large, so that a high order aberration can not be satisfactorily corrected.
The condition (5) provides an appropriate display image field size on the basis of the distance L between the first surface and the second surface. Below the lower limit of the condition (5), the lens has the diameter unsuitably large for the reduced image field size, which is undesirable. On the other hand, above the upper limit of the condition (5), the image field size becomes too small, resulting in difficulty in aberration correction.
According to the present invention, it is also preferable to satisfy the following conditions:
NA greater than 0.65;xe2x80x83xe2x80x83(6)
xe2x80x830.05 less than f2/f4 less than 6;xe2x80x83xe2x80x83(7)
0.01 less than f5/L less than 1.2;xe2x80x83xe2x80x83(8)
xe2x88x920.8 less than f4/L less than xe2x88x920.008; andxe2x80x83xe2x80x83(9)
xe2x88x920.5 less than f2/L less than xe2x88x920.005,xe2x80x83xe2x80x83(10)
where
NA: the maximum numerical aperture on the image side;
f2: the focal length of the second lens group;
f4: the focal length of the fourth lens group;
f5: the focal length of the fifth lens group; and
L: the distance between the first surface and the second surface.
The condition (6) provides an appropriate range for the maximum numerical aperture NA on the image side. According to the present invention, it is provided a projection optical system capable of obtaining a large numerical aperture even if the effective diameter of lens is small. As a result, below the lower limit of the condition (6), the effect of the present invention can not be fully obtained.
The condition (7) provides an appropriate range for a ratio between the refracting powers of the fourth lens group G4 of negative refracting power and the second lens group G2 of negative refracting power, in order to correct excellently a curvature of field while maintaining a wide exposure area by approximating a Petzval sum to zero. Below the lower limit of the condition (7), the refracting power of the fourth lens group G4 becomes relatively weak to the refracting power of the second lens group G2, so that a large positive Petzval sum is undesirably generated.
On the other hand, above the upper limit of the condition (7), the refracting power of the second lens group G2 becomes weak relative to the refracting power of the fourth lens group G4, so that a large positive Petzval sum is undesirably generated.
The condition (8) provides an appropriate range for the refracting power of the fifth lens group G5 of positive refracting power, in order to correct a spherical aberration, a distortion and a Petzval sum in a good balance while maintaining a large numerical aperture. Below the lower limit of the condition (8), the refracting power of the fifth lens group G5 becomes too large, so that not only a negative distortion, but also negative spherical aberration are generated to be large in the fifth lens group G5 undesirably. On the other hand, above the upper limit of the condition (8), the refracting power of the fifth lens group G5 becomes too weak, and the refracting power of the fourth lens group G4 of negative refracting power inevitably becomes weak correspondingly. As a result, it becomes impossible to correct a positive Petzval sum satisfactorily.
The condition (9) provides an appropriate range for the refracting power of the fourth lens group G4 of negative refracting power. Below the lower limit of the condition (9), it becomes undesirably difficult to correct a spherical aberration. Conversely, above the upper limit of the condition (9), a coma aberration is generated undesirably. In order to correct a spherical aberration and a Petzval sum satisfactorily, it is preferable to set the lower limit of the condition (9) to xe2x88x920.078, and it is preferable to set the upper limit of the condition (9) to xe2x88x920.047 to further suppress generation of a coma aberration.
The condition (10) provides an appropriate range for the refracting power of the second lens group G2 of negative refracting power. Below the lower limit of the condition (10), a Petzval sum becomes a large positive value undesirably. On the other hand, above the upper limit of the condition (10), a negative distortion is undesirably generated. In order to further correct the Petzval sum satisfactorily, it is preferable to set the lower limit of the condition (10) to xe2x88x920.16. In order to further correct the negative distortion and the coma aberration satisfactorily, it is preferable to set the upper limit of the condition (10) to xe2x88x920.071.
Next, in the present invention, it is preferable to provide at least one negative lens in the fifth lens group G5. With this arrangement, a distortion can be excellently corrected.
It is also preferable to provide in the fourth lens group G4 of negative refracting power at least two pairs of concave lens surfaces facing each other. With such arrangement, a light beam can be loosely bent, so that generation of especially a spherical aberration can be suppressed.
In the same manner, it is preferable to provide in the second lens group G2 of negative refracting power at least two pairs of concave lens surfaces facing each other. With such arrangement, a light beam can be loosely bent, so that generation of especially an off-axis aberration can be suppressed.
In the same manner, it is preferable to provide in the fifth lens group G5 of positive refracting power at least one pair of convex lens surfaces facing each other. With such arrangement, a light beam can be loosely bent, so that generation of especially a spherical aberration can be suppressed.
In the same manner, it is preferable to provide in the third lens group G3 of positive refracting power at least one pair of convex lens surfaces facing each other. With this arrangement, a light beam can be loosely bent, so that generation of especially an off-axis aberration can be suppressed.